"Correct" way to add texture for a photo print in a scene. Fix "over-exposure"
Ok, here's what I ended up doing. Thank you @virgil-sisoe for the pointers.

First up, I may have been overly concerned about gamma correction: it looks like Blender automatically gamma "uncorrects" (wrongs?) texture data when it's loaded, so the output of the texture node can be assumed to be linear.

From there I added a "Separate RGB" node, and added a Multiply node on each of its 3 outputs, followed by a "Combine RGB" node. I set each of the multiply nodes to 0.5, so -1 stop.

(arguably it should have been -1.6 stops, but -1 looked right given the brightness of the rest of the scene).

I added gamma nodes to the input and output, but have the input gamma set to 1 (so no gamma adjustment), and the output a little over 1 to add just a touch of contrast.

The last thing I did was to switch from the principled BSDF shader to the diffuse BSDF shader. I figured that I only want diffuse here, and I don't yet fully understand the principled shader.

This looks ok, and I think it will suffice for my current purposes.

Here's what I ended up with: